This invention relates to a color ratiometric optical scanner for sorting small particles such as beans and seeds. Such articles must be sorted on an individual basis with great discrimination accuracy in order to detect the various blemishes and color irregularities that exist. Since the aforementioned articles have a very low unit cost it is essential that the discrimination accuracy be coupled with high speed operation in order to make color sorting of these articles economically feasible.
Light detecting and colorimetric methods have been used for some time to discriminate the color or reflected light of various articles such as fruit, nuts, beans, tiles, roasted peanuts, etc. Information from previous optical heads has been used to sort, reject or control, for example, the color of roasted peanuts. Much of the apparatus to date has been relatively complex and expensive. Also previous sorting devices have suffered because the particles are usually not of uniform size and therefore past attempts at providing light detecting means for sensing blemishes and color irregularities usually have had the disadvantage of not being able to discriminate size variations from color and blemish variations. Other difficulties affecting the accuracy of previousely used devices have often been such common problems as optical referencing and electronic drift and noise. Whereas noise performance must be designed in, problems of drift and instability of control has often been overcome only at the time-consuming penalty of interrupting the sorting operation to accommodate periodic readjustment, nulling or referencing.
In the sorting of the aforementioned products, it is necessary to discriminate between various types of defects such as discoloration, water or stain damage, which discolors the entire product, as well as very small blemishes and discolored areas which affect only a portion of the product. Additionally, it is necessary to allow for the fact that some products may have a variation in color on a portion of the product such as the "black eye" in a blackeyed pea, which can in currently known high speed sorting apparatus cause that object to be labeled as an undesirable product when in fact the discoloration is a normal condition. For products that vary in size but are sorted for color criteria the color ratiometric system is the best since it is not size sensitive.
However, one major problem with color ratiometric sorting is that the color shades that must be discriminated are usually subtle. In a color ratiometric system, two regions of the electromagnetic spectrum are chosen to provide the maximum discrimination. The energy from one spectral region is divided by the energy from another spectral region to provide a value that is used for control or comparison against a standard for threshold information which is the basic for acceptance or rejection of the product being tested. When color hues are subtle (such is generally the requirement when grading beans for instance), variations of approximately one percent (1%) in the ratio represents the acceptable threshold drift allowed inasmuch as manual ratio-threshold adjustments on the order of one percent (1%) often make the difference between economically profitable and unsatisfactory sorting operations when the product upgrading and wastage of these articles is considered.
Thus it can be seen that relative drift in the gain of the photo channels amounting to more than one percent (1%) is prohibitive. There are several causes of relative gain drift between channels and the most offensive is the optical detector's sensitivity drift. Other causes are lamp color temperature variations (usually due to lamp aging) and the analog divider drift, especially occurring if the log-antilog semiconductor variety is used. Using present day precision resistors and high gain operational amplifiers the detector amplifier gain drift is less of a problem and gain stability of 1 part in 1000 is obtainable. In prior systems, photo multipliers and cadmium sulphide optical detectors have been used. Photo multipliers have high sensitivity but suffer a constant gain degradation that does not necessarily match between any two of them. Also, the requirement for an extremely stable dynode voltage supply is a significant cost factor in their use. Cadmium sulphide cells suffer "light history" effects and exhibit temperature related gain variations that also do not necessarily track between similar units.
In order to compensate for these gain variations just discussed, many existing systems compare against a standard. The standard is usually a color background that does not vary. This color background is measured on a regular basis (perhaps between every object being viewed) and a gain correction is made. While this is an ideal method of gain compensation, the logistics of placing a standard object in the stream of objects being sorted is a difficult mechanical problem. Therefore, when a color standard must be used, it is most commonly implemented by placing a color background standard placard opposite each photodetector used so that each detector views the placard between articles being sorted. The use of such background reference placards however imposes limiting geometric restrictions on such apparatus since each placard must fill the entire field of view of its related photodetector if extraneous color information is to be excluded.
To compensate one must employ a small number of wide-viewing detectors or else a larger number of narrow-viewing detectors since the mounting positions reserved for placards cannot be used for photocells as well. In either case it is not possible to use a large number of detectors with widely over-lapping fields of view if color background standards are required for gain stability. Furthermore, the modified gain information must be electronically stored since the background reference signal and product signal do not occur at the same time. An additional and serious objection to the use of background color standards is their inevitable degradation due to the dust, dirt and smudging common to the warehouse environment. Another common method to compensate for drift, etc. is to optically chop the signals and have a single detector channel. Again, this is an ideal gain compensation method but the mechanical complications of chopping the optical signal especially when the optical field of view should be spherical makes for a very complex optical head. Chopping also imposes a band width limitation that often limits the rate at which items may be sorted.
Because of the complexity of employing the methods described above, prior art optical heads generally have not viewed the entire surface of the product being sorted, thus missing marks and blemishes. A more pronounced failing of previous systems is the inability to allow for the common marks which are characteristics of the product, i.e. the red spot on a "red-eye bean". If the entire surface of the bean is not viewed, the spot, depending on the bean orientation, will or will not be seen by the field of view thereby causing a variation that should not be included in the color measurement criteria. In addition to standard color backgrounds, prior art apparatus have also utilized fairly precise optical slits placed in front of the photo detectors which then requires sophisticated lens systems and additionally severely limits the area of the photo detector viewing the objects, thus reducing the signal level, making it more susceptible to electronic noise.
Thus it can be seen that prior art apparatus in order to be economically feasible and practicable will generally be a compromise in design between extreme complexity requiring low operator skill as well as maintenance, and less complex design coupled with higher operator skill requirements and additional maintenance, setup and adjustment requirements. Since users of this type of sorting apparatus rarely operate said apparatus under conditions which approach laboratory conditions, i.e. a great deal of dust and dirt is fairly common when sorting the aforementioned products, and further since the labor force available to the users are generally of a lower than average technical skill level and sophistication, it can be seen that it is generally difficult to obtain operators that possess the skills required to set, adjust and maintain the sophisticated apparatus currently available and described in prior art disclosures. Additionally, because of the complexity of the apparatus and the lack of skilled operators, a substantial amount of hand sorting is required after machine sorting in order to render the product marketable. Since the sorting of said products is generally a low profit margin operation, losses of marketable product due to improper setting of the aforementioned apparatus cannot be tolerated. Users of such apparatus are, therefore, constantly searching for designs which require less skilled operators, pose fewer setup and maintenance problems and are relatively insensitive to the dust and dirt encountered in normal operation.
It is, therefore, the primary object of this invention to provide a compact, economical light and color detecting scanner for accurately detecting very small as well as larger diffuse color irregularities and blemishes in articles, and which is capable of recognizing and allowing for normal localized color variations in certain articles, which device is insignificantly affected by the size of the articles being sorted or by dust deposits encountered during normal operation and which does not require operator adjustments for the sorting of different products.
A further object of this invention is to provide a light detecting scanner which minimizes photo detector drift, eliminates optical referencing as well as limiting the field of view of the photo detectors in a simple, compact, economical optical assembly specifically adapted for, but not limited to, the sorting of bean seeds and nuts.